If these backups fail, it is usually the 'VSS System Writer' which causes trouble.

You can find descriptions how to fix/re-register these from MS.

  - For W2K8 this always works for me

  - For W2K3 this might not work - then a reboot usually dos the job.

Since vssadmin list writers shows no error, vss writers are probably
not the cause.

Try restarting the volume shadow copy service and make sure
there is enough disk space to create shadow copies on the client.

Related thread:

https://community.emc.com/message/642254